I cannot reproduce the issue. IMO it has been fixed.

Moreover, you must join timers using timer.join(): a timer remains a thread.

Code:
---
import os
import threading
os.system(f"grep ^VmRSS /proc/{os.getpid()}/status")
# warmup
for n in range(10):
    timer = threading.Timer(5, None)
    timer.start()
    timer.cancel()
    timer.join()
os.system(f"grep ^VmRSS /proc/{os.getpid()}/status")
for n in range(1000):
    timer = threading.Timer(5, None)
    timer.start()
    timer.cancel()
    timer.join()
os.system(f"grep ^VmRSS /proc/{os.getpid()}/status")
---

Output on Linux with the main branch of Python (3.11):
---
VmRSS:	   10924 kB
VmRSS:	   11104 kB
VmRSS:	   11104 kB
---